Background

The United States promoter NEXTERA ENERGY RESOURCES, one of the leading operators in solar energy generation in the American continent owns the thermo-solar power plants TERMOSOL 1 and 2, built in the municipality of Navalvillar de Pela (Badajoz, Spain) together with one of the worldwide benchmark, Spanish engineering firms SENER at the head, where the company AKO has carried out the engineering project, supply and installation of its AKOTRACE® global electrical heat-tracing solution in both the molten salts and in the oil (HTF) and steam (BOP) parts.

Project Challenge

Technical requirements, although largely already resolved by AKO in many projects executed in thermo-solar plants with parabolic trough technology and salt (TES), oil (HTF) and steam (BOP) accumulation, included such important aspects for the correct operation of the plant:

Guaranteeing process working and maximum temperatures of materials in this type of plants with a risk of fire and explosion, in the circulation of the oil flow.

Optimising the energy cost so that the heating cables are connected for just enough time to compensate heat losses depending on the ambient temperature.

Tracing of critical points such as exchangers, vertical pipelines, valves and bonnets where due to the little available space and high power required, customised designs are necessary to guarantee the breaking of thermal bridges and heating of the system in specified times.

Integration of specific redundant control and protection systems for the salt processing area (TES), to protect the pipeline drainage circuits and tank against temperatures higher than those allowed.

The scope and capacity of AKO's management in the simultaneous execution of two complete plants, that represent a total of 100 MW, applying its knowledge and experience and guaranteeing the efficiency and profitability of the two installations.

Characteristics and operation of the thermo-solar Power Plant

These thermo-solar power plants are sustainable electric production facilities that use the sun as a clean energy source, by means of the solar field of parabolic trough collectors, that represent the hot spot of the cycle, using concentrated solar power (CSP) techniques . These plants contribute to energy safety as they use a local resource and do not emit greenhouse gases.

The main characteristic of this type of plants is their parabolic trough mirror technology together with the use of specific fluids for transferring solar/thermal energy and its respective accumulation. Tracing is critical auxiliary service for the plant, as it guarantees maintaining the temperature of the fluids, essential for its operation.

Specifically, the operation of the parabolic trough reflectors consists of turning around an axis, tracking the sun, while at the same time concentrating solar radiation in receiving tubes. A heat capturing fluid (synthetic oil) circulates inside the receiving tubes that is heated by the solar rays concentrated in the tube up to a temperature of around 400 ºC.

The heat transferring oil (HTF) is used to generate steam overheated at high pressure which by means of its turbining in a Rankine cycle, it becomes electric energy. The heat transferring oil is also used to be exchanged with molten salts, that result in an excellent heat accumulator, hence capable of generating overheated steam when there is not enough solar radiation. With an amount of salts of around 30,000 tonnes 1,000 thermal MWh of energy can be stored.

As can be inferred from the plant's operating description, the pumpability of the related fluids is essential for the generation of electric energy and for maintaining all the facilities. This gives the tracing system a particularly critical character, as it is in charge of the maintenance of conditions of the entire system.

AKOTRACE® Solution

Electrical heat-tracing systems are one of the main devices in the industrial heating or temperature maintenance market. This is a proposal suitable for large and small plants with a very low maintenance cost, guaranteeing the safety and reliability of their operation, given their great robustness and resistance to adverse weather conditions and unforeseen mechanical stresses they guarantee the maintenance of an even and controlled temperature as well as being subjected to thorough controls and periodic tests, according to current international standards.

The AKOTRACE® electrical heat tracing solution AKO has installed in the TERMOSOL 1 and 2 thermo-solar power plants allows the maintaining of the temperature of both the heat transferring fluid (HTF) and the molten salts of the accumulation system (TES) above their freezing point, in periods of low or zero solar radiation and/or in periods in which no electric generation occurs. The AKOTRACE® solutions guarantees a zero operating risk in the plant, as it prevents the freezing of the fluids required for its operation with several safety factors. Specifically, it guarantees the pumpability of both the molten salts in the accumulation system (raising them to temperatures of 265 ºC) and the thermal oil.

The AKOTRACE® solution also allows minimising the electric energy required for protecting against freezing, which allows the production of both plants to be energetically efficient.

Results

The difference provided by the AKOTRACE solution for this two plant project is that technical innovations have been introduced in the AKOTRACE® solution, for both electrical heat tracing and control systems and power and control cabinets, guaranteeing the process in the salt concentration part and in oils (HTF) and steam (BOP) and reducing the investment costs.

· Optimisation of the mineral insulation cable installation for the salt area (TES) and oil (HTF) and the self-regulating cable for the steam area (BOP) and its regulation in singular points: valves, bonnets and supports.
· Improvement of the control algorithm to prolong the durability of the mineral insulation heating cable.
· Increased functions of the SOFTrace software for better analysis and diagnosis of the status of the circuit, and to make the start-up and operating process easier.
· AKOTRACE® control system based on a new PLC platform with more features and availability.
· Construction improvements and improved design of the PROTrace electric power distribution cabinets.

Work in the two facilities has been overlapped in time, highlighting AKO's main qualities, customer proximity, which has provided it with all the necessary engineering documentation, flexibility, taking part in managing the supplies and, in short, the capacity for carrying out the different works simultaneously.

AKO started the Termosol 1 and 2 Thermosolar Power Plants project in November 2011 and finished the last one in March 2013, with a power of 50 MW, preventing the emission of 149,000 tonnes of CO2 a year. The power plant has been built on 470 hectares of land with over 624 collectors. The plant supplies the grid with enough energy for the consumption of 60,000 households.

About AKO

Over 30 years of innovation and with 130 people team of in-house professionals AKO Electromecànica, S.A.L. is a Spanish company leader in the Regulation and Control of Temperature with Electrical Heat Tracing Solutions for Concentrated Solar Power Plants (CSP), that offers comprehensive support from project design and manufacturing, to technical assistance, to the System commissioning (turnkey projects) and after-sales service. Electrical Heat Tracing System AKOTRACE® based the heating cable technology, providing advantages and savings on energy and the overall cost of the installation, with a reliable and secure management, ensuring the health, safety and respect for the environment.